Catalytic cracking is a petroleum refining process, which is applied commercially on a very large scale. A majority of blending pool (gasoline/TCO) is produced using FCC process. In the process, heavy hydrocarbon feed stock is converted into lighter products by reactions taking place at elevated temperature in the presence of catalyst, with majority of conversion takes place in vapor phase. The feed stock is converted into gasoline, distillate & other liquid cracking products as well as lighter gaseous cracking products of four or less carbon atoms per molecule. The gas partly consists of olefins and partly of saturated hydrocarbons.
During cracking reactions some heavy material known as coke, is deposited onto the catalyst. This reduces its catalytic activity and regeneration is desired. After removal of occluded hydrocarbons from spent cracking catalyst, regeneration is done by burning off the coke to restore the catalytic activity. The three characteristics steps of a typical catalytic cracking process can be identified as follows:                A cracking step in which the hydrocarbons are converted into lighter products,        A stripping step to remove hydrocarbons adsorbed on the catalyst and        A regeneration step to burn off coke from the catalyst.The regenerated catalyst is then used in the cracking step.        
Catalytic cracking feedstocks normally contain sulfur in the form of organic sulfur compounds such as mercaptan, sulfides and thiophenes. The products of the cracking process correspondingly tend to contain sulfur impurities even though about half of the sulfur is converted to Hydrogen Sulfide during cracking process, mainly by catalytic decomposition of non-thiophinic sulfur compounds. Although the amount and type of sulfur in the cracking products are influenced by the feed, catalyst types, additives, conversion and other operating conditions, a significant portion of sulfur generally remains in the product pool. With increasing environmental regulations being applied to petroleum products, the allowable sulfur content of the products has generally been decreased in response to concerns about the emissions of sulfur oxides and other sulfur compounds into the air following combustion processes.
One approach has been to remove the sulfur from the FCC feed by hydrotreating, before cracking is initiated while highly effective, this approach tends to be expensive in terms of the capital cost of the equipment and also since hydrogen consumption is high. Another approach has been to remove the sulfur from the cracked products by hydrotreating. Again while effective, this solution has the drawback that valuable product octane may be lost when the high octane olefins are saturated.
From an economic point of view, it could be desirable to achieve sulfur removal in the cracking process itself since this would effectively de-sulphurize the major components of gasoline, diesel without additional treatment. Various catalytic materials have been developed for the removal of sulfur during FCC process cycle but so far most developments have been centered on removal of sulfur from the regenerator stack gases. The sulfur is removed from stack gases from regenerator but product sulfur levels are not greatly affected. An alternative technology for removal of sulfur oxides from regenerator, removal is based on the use of magnesium aluminium spirids as additives to the circulating catalyst inventory in FCCU.
Exemplary patents on this type of sulfur removal additives include U.S. Pat. Nos. 4,963,520; 4,957,892; 4,957,718; 4,790,882 etc. Again product sulfur levels are not greatly reduced.
A catalyst additive for reduction of sulfur levels in the liquid cracking products is proposed by Wormsbecher and Kim in U.S. Pat. Nos. 5,376,608 and 5,525,210 using a cracking catalyst additive of an alumina supported Lewis acid for the production of reduced-sulfur gasoline, but this system could not achieve commercial success.